High temperature is a major factor limiting the growth of cool-season plant species during summer. Understanding mechanisms of plant tolerance to high temperature would help develop effective management practices and heat-tolerant cultivars through breeding or biotechnology. This dissertation research explored physiological, biochemical and molecular mechanisms for improving heat tolerance in two bentgrass species, creeping bentgrass ( Agrostis stolonifera L.), a widely used cool-season grass species on golf course tees and putting greens, and thermal rough bentgrass ( Agrostis scabra Willd.) adapted to geothermal areas in Yellowstone National Park.

The dissertation reports research in three main components. The first section compared differential heat-induced metabolism of hormones, proteins and metabolites between heat-sensitive creeping bentgrass and heat-tolerant A. scabra. Based on the findings that heat tolerance of bentgrass was associated with changes in stress-related hormone levels, the effects of foliar-applied hormone or hormone inhibitors on creeping bentgrass to enhance heat tolerance were further studied. Results from both growth chamber and field studies confirmed the effectiveness of applying hormones or hormone-based plant growth regulators on alleviating heat injuries in creeping bentgrass. In the last part of the dissertation, a few transgenic creeping bentgrass lines with improved heat tolerance were characterized. These transgenic lines carry a gene ( ipt ) controlling cytokinin synthesis. Increased ipt gene expression and cytokinin levels were confirmed and changes in morphological and physiological traits of the plants were examined. Genome-wide protein responses to the addition of the gene and their association with heat tolerance were discussed. The results indicated that transformation with the ipt gene induced protein changes involved in multiple functional groups, mainly in energy, protein destination and storage, and disease/defense categories in both leaves and roots of creeping bentgrass, thus cytokinins may have regulatory roles in multiple metabolic pathways for heat tolerance. Taken together, these studies suggest summer performance of creeping bentgrass may be improved by properly applying hormone-based plant growth regulators or biostimulants, and incorporating molecular markers developed from heat- and/or hormone-responsive proteins and metabolites may facilitate selection of heat-tolerant creeping bentgrass cultivars.